The metal that is extracted from sea water is

Which of the following,on thermal decomposition,form both acidic and basic oxides along with $O_2$?
$(i)$ $NaNO_3$ $(ii)$ $Ca(NO_3)_2$ $(iii)$ $Be(NO_3)_2$ $(iv)$ $LiNO_3$
The correct option is

Which of the following compounds is formed when sodium burns in excess supply of air?

With an increase in atomic number,the melting point of alkali metals $...$

Correct order of solubility is

State as to why:
$(a)$ a solution of $Na_2CO_3$ is alkaline?
$(b)$ alkali metals are prepared by electrolysis of their fused chlorides?
$(c)$ sodium is found to be more useful than potassium?